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Cut the cauliflower into bite-sized pieces. Reserve the leaves, except any tough ones. Put the stalk pieces in a microwave-safe bowl, then the florets. Add 1 tbsp water and cover. Microwave for 8 mins until tender, then remove and drain, if needed. Transfer the cauliflower to another microwave-safe dish.
Tip the cornflour and a splash of the milk into the bowl used to cook the cauliflower, and mix to a smooth paste. Whisk in the rest of the milk. Season. Stir in the mustard, thyme and cheddar, then pour this over the cauliflower. Top with the cauliflower leaves and dot over the butter. Microwave on high for 5 mins until the cauliflower is tender and the cheese has melted. Will keep chilled for up to 24 hours. Leave to cool first. Sprinkle with a little extra cheese, if you like, and finish under a grill or in a combi microwave until golden.